1. Field of the Invention
The present invention relates to a semiconductor memory device using a nonvolatile memory, a method for driving the semiconductor memory device and a portable electronic apparatus.
2. Description of the Related Art
Conventionally, a flash memory is typically used as a nonvolatile memory.
In a flash memory, as shown in FIG. 24, a floating gate 902, an insulating film 907 and a word line (control gate) 903 are formed in this order on a semiconductor substrate 901 via a gate insulating film. On both sides of the floating gate 902, a source line 904 and a bit line 905 are formed by a diffusion region, thereby constructing a memory cell. A device isolation region 906 is formed around the memory cell (see, for example, Japanese Unexamined Patent Publication No. Hei 5-304277 (1993)).
The memory cell retains data according to a charge amount in the floating gate 902. In a memory cell array constructed by arranging memory cells, an operation of rewriting/reading a desired memory cell can be performed by selecting a specific word line and a specific bit line and applying a predetermined voltage.
In such a flash memory, when a charge amount in the floating gate changes, a drain current (Id)-gate voltage (Vg) characteristic as shown in FIG. 25 is displayed. When the amount of negative charges in the floating gate increases, the threshold increases, and an Id-Vg curve shifts substantially in parallel in the direction of increasing Vg.
In such a flash memory, however, it is necessary to dispose the insulating film 907 for separating the floating gate 902 and the word line 903 from the functional viewpoint. In addition, in order to prevent leakage of charges from the floating gate 902, it is difficult to reduce the thickness of the gate insulating film. It is therefore difficult to effectively reduce the thickness of the insulating film 907 and the gate insulating film, and it disturbs reduction in size of the memory cell.